1. Field of the Invention
The present invention relates generally to ram air turbine devices and more particularly to a ducted type ram air turbine generator.
2. Description of the Prior Art
One type of ram air turbine generator disclosed in the prior art consists of a ram turbine generator with the blades mounted externally to extract power from an air stream. The blades are usually mounted to a rotatable housing forming part of a center aerobody with the turbine center shaft drivingly coupled to an electric generator, or a hydraulic pump, or both if desired. Turbine speed control and power output are maintained through speed control mechanisms which vary the pitch of the blades under varying flight conditions, thereby tending to maintain constant power to the blades from the air stream. This type of ram air turbine generator is presently the predominate type used on externally carried electronic pods, primarily in military applications, and for emergency power electric/and or hydraulic power. The units are stored in the wing or fuselage of an aircraft and are deployed into the air stream when there is failure of the onboard aircraft power. Recent patents relate to improvements in this basic technology. U.S. Pat. No. 5,249,924 (Brum) relates to mechanisms and controls to adjust the pitch of the blades for speed control. U.S. Pat. No. 4,991,796 (Peters, et al) discloses a drive system between a ram air turbine and power generator units aboard a host aircraft. U.S. Pat. No. 5,122,036 (Dickes et al) discloses an externally bladed ram air turbine generator with a mechanism to prevent blade stall and to allow power generation at low speeds, including aircraft landing approach and final landing.
The process of converting air stream power to mechanical rotary power in externally bladed turbines, in terms of power extracted relative to the power dissipated through drag, is relatively efficient at low to moderate subsonic speeds. However as the flight speed of the aircraft increases, to values where the relative velocity between the air and blade becomes sonic, efficiencies of the process fall off dramatically. In these regimes the shock waves, induced by the blades, create high frontal pressures on the blades and flow separation over and behind the blades, with corresponding dramatic increases in drag. This can occur at flight speeds in the range of Mach=0.60 depending on the blade design and shaft rotational speed. As flight speed further increases to high subsonic, through transonic, and into supersonic flight regimes, the drag to lift ratio can further increase at least several times depending on blade profile, reflecting equal increases in the drag for the same power extracted from the air stream, which is produced by the blade lift. These drag increases can be critical in applications to electronic pods mounted to high performance supersonic fighter aircraft. With increasing power needs for electronic systems in external aircraft pods, this drag penalty for the external turbine bladed technology has increasing undesirable impact on aircraft performance, adversely affecting speed, maneuverability and range. In addition, external bladed ram turbine generators do not have the capability to provide direct cooling from the exhaust air. In applications where portions of the flight profile include high speed flight which induces significant aerodynamic heating on the skin of a pod, additional active cooling systems are required for the pod electronics, entailing additional size and weight penalties.
Ducted ram air turbine generators are a second type of ram air turbines. U.S. Pat. No. 4,267,775 (Sjotun) discloses a ram air turbine generator positioned internally, in the nose of a missile, with a wreath arrangement of inlet ducts supplying ram air to the inlet of a radial flow turbine. Outlet ducts direct the exhaust flow forward. During supersonic flight of the missile, the shock waves off the front of the missile increase the pressure in front of the exhausts, tending to increasingly throttle the flow through the ducts and turbine as the missile accelerates, thereby tending to limit the maximum speed of rotation of the turbine. However, drag reduction features were not a goal and were not present. The drag is large for the power extracted, due to the reaction forces set up from the full reversal of the air inlet stream, and the resistance to the exhaust flow from the oncoming air stream. No direct cooling capability is provided.
U.S. Pat. No. 4,477,039 (Boulton, et al) discloses a vented cowl variable geometry inlet for aircraft. A variable area vent, in the side of an air inlet cowl with a slidable door, can be positioned to allow air flow dumping, thereby permitting starting with high contraction inlets, for example as used in ram jets, and to control airflow to the engine during flight. The system is intended for use with supersonic aircraft air induction systems associated with air driven auxiliary power equipment. The system controls airflow supply to air driven power equipment, as is needed for speed and power control. However, the basic approach offered, including the air induction and then venting, with the shape of the leading edge of the door diverting the flow outward through vents, inherently does not offer a reduced drag.
Present inventor Ghetzler's earlier invention entitled "Ram Air Turbine Generating Apparatus", U.S. Pat. No. 5,505,587 discloses a ducted ram air turbine that obtains some measure of power output and speed control from purely aerodynamic and spring activated mechanical internal control elements. However, the ducted ram air turbine of that invention results in speed and power variation as great as thirty percent above or below a nominal design value throughout moderate subsonic through supersonic flight speed range. For many airborne applications the final power supplied to electronic systems requires a tighter tolerance on power and speed usually in the range of five percent. Thus, the use of this earlier invention may entail additional power conditioning systems with increasing size and weight penalties. This earlier invention has bypass features which admit and then bypass and exhaust a portion of the airflow before entering the turbine. As in the Boulton, et al, patent, the process of admitting, bypassing, and then exhausting airflow presents drag penalties due to the momentum interchange.
It is with the knowledge of the state of the present technology and limitations of that technology as just set forth, that the present invention was conceived and now has been reduced to practice.